High brightness green-emitting phosphor

ABSTRACT

A high brightness green-emitting phosphor is described. The phosphor has the general formula: 
     
       
         (Y (1-x-y-z) Gd x Tb y La z )BO 3   
       
     
     where 
     0.01≦x≦0.4, 
     0.01≦y≦0.1, and 
     0&lt;z≦0.2. 
     The phosphor exhibits an enhanced brightness under VUV excitation which is desirable for plasma display panels.

TECHNICAL FIELD

This invention is related generally to phosphors for plasma displaypanels. More particularly, this invention is related to green-emittingborate phosphors.

BACKGROUND OF THE INVENTION

In order to achieve full color reproduction, a plasma display panel(PDP) is composed of pixels which emit light corresponding to the threeprimary colors of red, green and blue. The pixels are formed from red-,green-, and blue-emitting phosphors which have been deposited in a pasteor ink form on an inner wall of the device. The binder materials arethen burned out at a high temperature (about 400° C.). The tri-coloremission is achieved by stimulating the phosphors with vacuumultraviolet (VUV) radiation generated by Xe-Ne gas discharges (172 nm).The luminous efficiency, color chromaticity, afterglow, and stability ofthe phosphors have significant roles in the PDP performance. Typicalphosphors used in PDP applications include a red-emitting (Y,Gd)BO₃:Euphosphor (YOB), a green-emitting Zn₂SiO₄:Mn phosphor, and ablue-emitting BaMgAl₁₀O₁₇:Eu.

The conventional green-emitting Zn₂SiO₄:Mn phosphor suffers from a longdecay time and a decreasing luminous efficiency which negatively affectspanel characteristics. A potentially more effective green-emittingphosphor is yttrium gadolinium terbium borate, (Y,Gd,Tb)BO₃. Theluminescence properties of a Y_(0.95-x)Gd_(x)Tb_(0.05)BO₃ phosphorsystem in the VUV region have been described by Kwon et al. J.Luminescence 87-89 (2000) 1039-1041. Japanese Patent Application11-071581 to Ohto describes a green-emitting (Y,Gd,Tb)BO₃ for use inplasma display panels. The (Y,Gd,Tb)BO₃ phosphor exhibits a higher VUVabsorption at 172 nm and experiences a smaller decrease in brightnessafter binder burnout.

SUMMARY OF THE INVENTION

It has been discovered that substituting a small amount of lanthanum foryttrium improves the brightness of the (Y,Gd,Tb)BO₃ phosphor under VUVexcitation. The substituted phosphor has the general formula:

(Y_((1-x-y-z))Gd_(x)Tb_(y)La_(z))BO₃

where

0.01≦x≦0.4,

0.01≦y≦0.1, and

0<z≦0.2.

Preferably, 0.2≦x≦0.3, 0.03≦y≦0.07 and 0.05≦z≦0.15. More preferably,0.05≦z≦0.1. The phosphor exhibits a bright green emission (CIE colorcoordinates; x=0.323, y=0.610) under VUV excitation. However, increasingthe amount of lanthanum above the specified range causes a decrease inthe VUV brightness.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE illustrates the change in the VUV brightness as a function ofthe lanthanum concentration.

DETAILED DESCRIPTION OF THE INVENTION

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims taken inconjunction with the above-described drawing.

The following non-limiting examples are presented.

EXAMPLES

Several samples of yttrium gadolinium terbium borate phosphors wereprepared with increasing lanthanum substitutions. The phosphors have thegeneral formula (Y_((1-x-y-z))Gd_(0.245)Tb_(0.65)La_(z))BO₃. Thephosphors were made from a rare earth oxide co-precipitate formed bydissolving an amount of terbium oxide in 200 ml of concentrated nitricacid, diluting with 300 ml of water, and then heating at 80° C.-90° C.while stirring until the solution became clear. An additional 200 ml ofconc. HNO₃ was then added. Weighed amounts of yttrium oxide, gadoliniumoxide and lanthanum oxide were then added slowly to a portion of theterbium-containing solution. The second solution was stirred untilclear. An additional 50 ml of conc. HNO₃ was added if the solution didnot become clear within 30 minutes. A 2-liter quantity of an aqueousoxalic acid solution (1.7 moles oxalic acid to 1 mole rare earth oxide)was then added and the pH of the second solution was adjusted to 1-2with ammonium hydroxide (400 to 450 ml). The co-precipitate wasfiltered, dried and heated in an alumina crucible at 1100° C. for 8hours to form the oxides. The borate phosphor was prepared by combiningan amount of the calcined rare earth oxide co-precipitate in a 1:1 moleratio with boric acid and firing the mixture at 1060° C. for 3 hours ina 3% hydrogen atmosphere. The quantities of Y,La,Tb and Gd in moles permole of phosphor and the VUV brightness of these materials under 172 nmexcitation is given in following table.

% rel. VUV Example Y La Tb Gd brightness 1 0.69 0 0.065 0.245 100(control) 2 0.64 0.05 0.065 0.245 106 3 0.59 0.1 0.065 0.245 107.7 40.49 0.2 0.065 0.245 101 5 0.39 0.3 0.065 0.245 97 6 0.29 0.4 0.0650.245 88 7 0.19 0.5 0.065 0.245 83 8 0.09 0.6 0.065 0.245 76

As can be seen, the lanthanum addition causes an increase in therelative VUV brightness of the phosphor up to 0.2 moles of La per moleof phosphor. At 0.3 moles of La per mole of phosphor, the VUV brightnessbecomes less than the VUV brightness of the control which did notcontain lanthanum.

While there has been shown and described what are at the presentconsidered the preferred embodiments of the invention, it will beobvious to those skilled in the art that various changes andmodifications may be made therein without departing from the scope ofthe invention as defined by the appended claims.

We claim:
 1. A phosphor having the general formula;(Y_((1-x-y-z))Gd_(x)Tb_(y)La_(z))BO₃ where 0.01≦x≦0.4, 0.01≦y≦0.1, and0<z≦0.2.
 2. The phosphor of claim 1 wherein 0.2≦x≦0.3 and 0.03≦y≦0.07.3. The phosphor of claim 1 wherein 0.05≦z≦0.15.
 4. The phosphor of claim1 wherein 0.05≦z≦0.1.
 5. The phosphor of claim 2 wherein 0.05≦z≦0.15. 6.The phosphor of claim 2 wherein 0.05≦z≦0.1.